1. Field of the Invention
The present invention generally relates to restoring DC spectrum for analog television signals in direct conversion receivers.
More specifically, the present invention relates to correcting DC offset and 1/f noise, or other unwanted DC noise in analog TV signals.
2. Background Art
Direct-conversion, or homodyne receivers are popular for many communications applications because of their simplicity and low power. They do not require intermediate-frequency (IF) filters, which are often costly, and need only one frequency conversion stage and one local oscillator (LO). By comparison, dual conversion receivers necessarily require two frequency conversion stages, and two different local oscillators. Therefore, the dual conversion receivers consume more power and have a higher part count than direct-conversion receivers.
Direct-conversion receivers rely entirely on quadrature mixing to obtain the necessary image rejection. Single-conversion low-IF receivers for broadband communication systems, such as direct broadcast satellite (DBS), broadcast or cable television (CATV), may rely on some combination of quadrature conversion and pre-selection with tracking or switched filters. By comparison, double-conversion receivers for these applications rely more on IF filtering for image rejection. Because the IF frequencies can be fixed, very sharp filters, such as surface acoustic wave (SAW) filters, need to be used in the dual conversion receiver. However, these filters are relatively costly and prevent the implementation of a truly integrated single chip solution.
In relation to DC offset compensation, a disadvantage of direct-conversion receivers is that some point in the middle of the desired channel spectrum is down-converted to 0 Hz, or DC. In other words, the direct conversion translates useful information directly to DC. However, DC offset voltages and 1/f noise also co-exist at DC with the useful information, which impairs the signal-to-noise ratio around DC. Blocking capacitors (AC coupling) can be used to reduce or eliminate DC Offset voltages and 1/f noise. However, blocking capacitors necessarily introduce a null in signal energy at DC. Therefore, useful signal information at DC is eliminated along with the unwanted DC Offset by the blocking capacitor. The DC null is acceptable for some communications systems. However, the DC null is unacceptable for analog TV signals because it would degrade the video signal below that which a viewer would find acceptable.
What is needed is a technique to compensate for DC offset in a direct conversion receiver that does not null the signal energy during an active portion of the input signal, so as to enable direct conversion of analog television signals without impairing viewing quality.